1. Field
This disclosure concerns a landfill cover system, more specifically to a landfill soil cover system for preventing water infiltration into a landfill and gas emission from the landfill under all weather conditions (i.e., any humid, semi-arid and arid climates).
2. Background
Landfilling is one major solution to manage municipal solid wastes in the world. Landfill waste can generate large amount of leachate and landfill gas. Water infiltration increases the rate and the amount of leachate generation and the risk of groundwater contamination. Landfill gas (mainly consists of CO2 and CH4) emission intensifies global warming, air pollution and may even cause fire and explosions. It is necessary, therefore, to develop and design a cover system for preventing water infiltration into a landfill, and gas emission from it. Preventing water infiltration into a landfill will reduce leachate.
A landfill normally has both flat and sloping topography. Conventional landfill covers often consist of compacted clay barriers or composite liners, which is made of geomembrane and compacted clay. The design of a compacted clay aims to meet the hydraulic criteria of a landfill cover systems; however, long-term hydraulic performance of compacted clay diminishes with time due to desiccation cracking, differential settlement, and wetting-drying cycle effects. Field studies have shown that desiccation can induce severe cracking of unprotected clay barriers which can create preferential flow into landfill wastes. It has been suggested that composite liners may be substituted over compacted clay. However, composite liners are not only expensive but also more susceptible to construction damage or post construction puncture due to its thin layer. Moreover, shear resistance between the interface of composite liner and soil is usually lower than soil internal shear resistance. There have been many examples of landfill instability problems caused by weak geosynthetic interfaces. Furthermore, the maintenance costs of this type of composite barrier are relatively high and the service life of geomembrane is relatively short. Therefore, some alternative covers are considered and used.
One alternative system is evapotranspiration (ET) cover, which generally consists of a thick layer of fine-grained soil with plants. A modification of this type of monolithic ET cover is a capillary barrier, which has gained popularity in arid regions. A capillary barrier is an earth cover system, which consists of two soil layers; that is, a fine-grained soil overlying a coarse-grained soil. The fundamental principle of a capillary barrier is to make use of contrasting unsaturated hydraulic properties of the two different soils to minimize rainfall infiltration and to drain away any infiltrated water quickly. It has been demonstrated that a capillary barrier performs well in arid and semi-arid regions, but not in humid areas and saturated ground conditions.
Some researchers proposed adding an unsaturated drainage layer between fine-grained and coarse-grained soil layers in a capillary barrier to increase the lateral drainage capability, to make the capillary barrier system applicable to landfills in relatively humid areas. This kind of modified capillary barrier system consists of three soil layers with saturated water permeability increasing successively from the top to the bottom (i.e., particle size increases with depth), but it is still only applicable to relatively arid and semi-arid conditions. When cover soils reach nearly saturated and saturated conditions (i.e., wet conditions) under heavy or prolonged rainfalls, it is clear that this type of cover system cannot effectively prevent water infiltration into landfill waste. In addition, this kind of modified capillary system may be only feasible for a sloping surface of a landfill, as its performance on flat surface of a landfill is questionable since the lateral drainage capability of the bottom coarse-grained soil cannot fit its functionality effectively.
Another issue with this type of modified capillary barrier system and with conventional barrier systems is that they cannot prevent gas emission from a municipal waste landfill. Capillary landfill cover systems have been unable to prevent water infiltration and landfill gas emission under all weather conditions (i.e., at any humid and arid climates) in the long term, especially when cover soils are nearly saturated and saturated.
In general, no capillary landfill cover system has demonstrated a capability to prevent water infiltration and landfill gas emission under all weather conditions (i.e., at any humid, semi-arid and arid climates) on a long-term basis, especially when cover soils are nearly saturated and saturated.